RDBMS Fundamentals: Keys

29 November 2017

This is the first in a series of RDMBS fundamentals posts. Let’s talk about keys. There are lots of keys in database design. Let’s take a look at some of them.

Primary Key

If we have any experience at all, we should understand the concept of a primary key. A primary key uniquely identifies a row in a database table. This might be an incrementing value, a GUID, or a combination of one or more columns already existing within the table. This can be a single column or multiple columns. A primary key is always a super key (see below). In most tables, the primary key may be a surrogate key, but this is not required.

Natural Key

A natural key is a value that exists in the real world that is already unique. There are several existing natural keys in the world.

Physical addresses are unique. There is only one 233 S Wacker Dr, Chicago, IL 60606. (That’s the Willis/Sears Tower.) If there were more than one address, we would not be able to deliver mail.

Bank account number. Within a bank, an account number cannot be duplicated. Outside of a single bank, the combination of routing number and account number is guaranteed to be unique.

Vehicle identification number. VINs are required to uniquely identify a year, make, and model of car. Many times, the trim level is also encoded into the VIN, but this varies by manufacturer.

Super Key

A super key is a set of columns that uniquely identifies a row. Let’s look at the following table. Any super key could potentially become a primary key.

A

B

C

D

a1

b1

c1

d1

a1

b2

c2

d1

a1

b1

c2

d2

a2

b2

c3

d3

a2

b2

c4

d3

Every row is unique, so columns A+B+C+D is a super key. However, there are also other ways to uniquely identify a row. All of the available super keys are available below.

4 column super keys: A+B+C+D

3 column super keys: A+B+C, A+C+D, B+C+D

2 column super keys: B+C, C+D

1 column super keys:none

It is important to know that just because a super key exists, that does not mean it should be a key. For example, you may have a table where first_name + last_name is unique. However, this would never be considered a key, since we know from the real world that a name has no guarantee of being unique.

The applicable set theory for super keys means that any combination of columns that includes a super key is also guaranteed to be a super key. In the above example, B+C is a super key. This means that any combination of columns that includes B and C is also a super key. As expected, A+B+C, B+C+D and A+B+C+D are all super keys.

Candidate Key

A candidate key is a super key with the minimal number of attributes. In the example above, both B+C and C+D are candidate keys. They are eligible to function as primary keys.

Surrogate Key

A surrogate key is any time we use an artificial value to uniquely identify a row. Examples of surrogate keys include identities or UUIDs. A surrogate key has no business meaning. Ideally, surrogate key columns will be named table_name_key or table_name_id. I prefer _key variant, because then it should be incredibly clear that a value has absolutely no business meaning whatsoever. Sometimes IDs have a business purpose. Think of something along the lines of employee IDs, corporate asset IDs, or ID card numbers. These are values that have some type of real-world meaning outside of the confines of the database, even if they are randomly or sequentially assigned. If we migrate from an old system to a new system, the ID should travel to the new system.

Alternate Key

An alternate key is any super key that is not the primary key. Because it is a super key, it may be one or more columns. Using an alternate key in a SQL WHERE clause would be expected to return exactly one or zero rows. It would never return two or more rows.

Suppose we have a table like our example, and we make the primary key B+C. In this table, C+D is also a super key. Any query with a clause WHERE C=cx AND D=dx will return exactly one or exactly zero rows.

Conclusion

When it comes to keys, that’s about it. Hopefully understanding what the different types of keys are will help you with your next table design.